Tachycardias are pathological accelerations of the heart rhythm. They can be classified into two distinctive categories based on the focus of their origin. Thus, a distinction is made between supraventricular tachycardia (SVT), which originates in the atrium, and ventricular tachycardia (VT), which originates in the ventricle. There is a suitable mode of treatment for each of these tachycardia based on delivery stimulation pulses of electric energy.
For SVTs, there are several possible known modes of treatment ranging from the absence of specific treatment to emitting stimulation pulses having an energy limited to a few micro-joules.
For VTs, it is known to be sometimes necessary to apply an electric shock stimulus of a few joules as soon as possible after the phenomenon has been observed. Such electric shocks are usually unpleasant and painful for patients and must be applied advisedly.
It is, therefore, important, when analyzing cardiac signals in the event of detection of an acceleration of the heart rhythm, to distinguish whether the situation is that of SVT or VT to avoid unnecessary shocks.
French patent No. 2,598,920 describes a system which senses the cardiac signal in the ventricle and detects therefrom the accelerations and decelerations of the heart rhythm. According to the analysis of this rhythm and of the series of accelerations and decelerations, the device evaluates the presence of an arrhythmia susceptible of being treated.
European patent No. 360,412 bases its interpretation of the tachycardia type on a classification, into several adjoining regions, of the rhythms analyzed from the cardiac signal detected in the ventricle.
These two systems have the drawback of relying solely on the signals picked up in one single cardiac chamber. It is difficult for devices that only detect the occurrence of tachycardia from signals picked up in the ventricle to distinguish SVT and VT conditions.
U.S. Pat. No. 4,860,749 describes a device that picks up the signals representative of the cardiac activity in both the ventricle and the atrium. The device defines different types of tachycardia by means of an algorithm using several rate thresholds and comparison with delays introduced into the device for each patient.
However, due to the numerous tests that must be conducted, such an algorithm is difficult to use in an implantable device.